This trailer is a step upward from the first one.
Each character gets some spotlight, Hawkeye finally gets to shoot an arrow, and I really like that it's more a showcase of mood, action, characters and the plot is still being kept under wraps. To many trailers spoil a lot, if not most, plotpoints of the movie.

The Thor movie explained that the Asgardians are aliens - as is everything else from the Norse Mythos. I'm thinking Loki is bringing around a version of Ragnarok and that little serpent is a version of the Midgard Serpent.

Fin Fang Foom was impersonated by the Midgard Serpent in the Thor comic. I'm thinking Midgard Serpent is more likely since people would probably have a hard time not laughing at the name, Fin Fang Foom.

ETA. I think of movie Thor as more inter-dimensional being than alien.

I like the name change - Avengers Assemble makes the best sense for the first of what could be many films. And I like how the Hulk's face looks like it was based on the actor's face rather than just being random.